5.4 Extravehicular Activities

Author 5.4 Michael L. Gernhardt Joseph P. Dervay James M. Waligora Extravehicular Daniel T. Fitzpatrick Johnny Conkin Activities Introduction The space shuttle was designed to provide crew and cargo external life support hoses, which could reduce mobility, transit from Earth to low-Earth orbit and back for conducting present snag hazards, and reduce overall reliability. The a wide variety of scientific experiments; deploying and cap- shuttle EMU also was designed to incorporate modular turing of satellites and scientific spacecraft; and providing a gloves and boots, and the boots were designed to be means by which to transfer crew and equipment to and from compatible with foot restraints to support working in future space stations. Space shuttle extravehicular activities microgravity. (EVAs) were initially anticipated to be used only for contin- gencies and, thus, were focused on addressing a variety of mal- The Apollo suits were designed to operate at the lowest functions associated with the deployment of radiators and reasonable pressure to minimize differences in pressure antennas and the closing and latching of the payload bay doors. across the suit and to permit optimal crew member mobility and dexterity for performing EVA tasks. They also provided protection against decompression sickness (DCS) and Evolution of the Extravehicular hypoxia under nominal and off-nominal conditions by using Activity Suits lower-pressure secondary oxygen system regulators. The current U.S. spacesuit system, the shuttle/International Space Station (ISS) EMU, has a nominal operating pressure of 30 The shuttle spacesuit system, formally called the extrave- kPa (4.3 psia). The atmosphere in both the shuttle and the hicular mobility unit (EMU), was derived from the advanced ISS resembles that on Earth, at a pressure of 100 kPa (14.5 Apollo suit configuration. In the Apollo Program, most psia) and a composition of 79% nitrogen and 21% oxygen. EVAs took place on the lunar surface, where suit durability, This atmosphere represents a major departure from the low- integrated liquid cooling garments, and low suit operating pressure, high-oxygen-concentration environments aboard pressures (25.8 kilopascals [kPa] or 3.75 pounds per square previous spacecraft. This combination of cabin atmosphere inch absolute [psia]) were required to facilitate relatively and suit pressures required the development of special lengthy EVAs that included ambulation and significant protocols to protect against DCS, which can result when a physical workloads. Metabolic rates during those EVAs crew member shifts rapidly from the relatively high-pressure averaged 1000 BTU/hour, with peaks of up to 2200 cabin to a low-pressure suit without sufficient time for 8 BTU/hour. The Apollo spacesuit, which had more than 15 denitrogenation (the elimination of nitrogen in lungs and components, included a biomedical belt for capturing and tissues via breathing pure oxygen). DCS can result from gas transmitting biomedical data, urine and fecal containment phase separation and bubble growth in various tissues. systems, a liquid cooling garment, a communications cap, a Depending on the location and degree of gas phase modular portable life support system, a boot system, thermal separation, DCS symptoms can range from minor joint overgloves, and a bubble helmet with eye protection. The discomfort to serious cardiopulmonary or central Apollo astronauts left a 34.4-kPa (5-psia) 100% oxygen envi- neurological symptoms that can result in physical disability ronment in the lunar lander to perform as many as 3 EVAs or loss of life. per mission on the lunar surface. The shuttle EMU incorporated many of the same components as the Apollo suit In addition to the risk of DCS, several other environmental system, but, like the shuttle vehicle, the suit was designed to and physiological stresses must be addressed or controlled be reusable. for crew members to safely undertake EVAs. These include, but are not limited to: thermal extremes that range from - The shuttle EMU incorporated a hard upper torso design with 120°C to +120°C, depending on whether the crew member is a range of arm and leg sizing elements that allowed the suits working during the day or at night; the need to provide to be resized for a variety of crew members of different body sources of energy (as much as 200 kcal/hour) to keep up with sizes; the Apollo suits, by comparison, were custom fitted. the metabolic energy expenditure on spacewalks, which can The hard upper torso design also eliminated the need for last 8 hours or longer; the need to provide hydration (as much 1 2 Michael L. Gernhardt, Joseph P. Dervay, James M. Waligora, Daniel T. Fitzpatrick, and Johnny Conkin as 1 liter/hour) and appropriate waste management systems; metabolic rates as high as 1600 BTU/hour, but the previous and the need to provide protection from the radiation, DCS trials had been conducted at work rates of micrometeoroid, and orbital debris environments. The approximately 800 BTU/hour that accurately simulated operating pressure of the suits, as noted above, is 29.6 kPa average EVA work rates. The pilot study had a crossover (4.3 psi), or approximately the same inflation pressure as a design between normal simulated EVA activity and an basketball; this also produces biomechanical constraints in activity profile that included 1 hour of 1600 BTU/hour. Work mobility and dexterity that can result in significant suit- performed during the second hour consisted of nominal induced trauma to the shoulders, wrists, and fingernails as activities. The high-intensity exercise protocol was a zero- well as trauma from point contact loads as the crew member prebreathe protocol with a final suit pressure of 44.8 kPa (6.5 moves within the stiff, pressurized suit. psi); it resulted in the same R value as the space shuttle protocols without engendering the need for the time-intensive oxygen prebreathe. The hypothesis was that high-intensity Preventing the Development of exercise would increase symptoms and bubbles; however, the Decompression Sickness only remarkable finding of that crossover study was that the last 2 high-intensity exercise runs done on consecutive days resulted in Type II symptoms, causing the tests to be Protecting against DCS has critical operational implications terminated. Subsequent analyses suggest that a zero-pre- because of the time needed for oxygen breathing to facilitate breathe protocol starting at a pressure of 1 atmosphere (atm) nitrogen elimination before an EVA can be safely performed, may make subjects particularly susceptible to Type II DCS and because of the medical and operational consequences of because the nitrogen elimination half-time of the brain and a DCS episode should one occur. spinal cord is 5 to 7 minutes; hence, an oxygen prebreathe would quickly desaturate the neurological tissues and The shuttle decompression protocols were developed based eliminate the autochthonous bubble formation that produces on findings from several hundred decompression trials that central nervous system symptoms of DCS. The zero- had been performed over a 10-year period. Two protocols prebreathe protocols, in contrast, would begin with supersat- were accepted for shuttle flight operations based on an “R” uration in the brain and spinal cord, even with R values value of 1.65. (The R value is the ratio between the nitrogen equivalent to those of protocols that incorporated some tension in a 360-minute half-time tissue compartment and the degree of oxygen prebreathe. spacesuit pressure.) These protocols consisted of either a 4- hour oxygen “prebreathe” performed while wearing a suit pres- Crews and flight planners preferred to use the 70.2 kPa (10.2 surized to 101.2 kPa (14.7 psia), or a staged decompression psia) staged decompression protocol for operational timeline protocol. In the staged decompression protocol, the crew reasons; consequently, all but 4 EVAs from the space shuttle members “prebreathe” oxygen for 1 hour before the cabin is used this protocol. Also, for operational reasons, the time depressurized to 70.2 kPa (10.2 psia) with 26.5% oxygen. spent at 70.2 kPa (10.2 psia) was typically far in excess of the The entire crew remains at 70.2 kPa (10.2 psia) for a mini- tested 12-hour exposure. Figure 5.4-1 shows the distribution mum of 12 hours, and then the EVA crew members complete of the duration at 70.2 kPa (10.2 psia) during the space another 75 minutes of oxygen prebreathe in the suit before shuttle missions that involved EVAs; it is apparent from this performing the EVA. The final in-suit prebreathe times are a figure that the average time spent at this pressure was over 40 function of the time spent at 70.2 kPa (10.2 psia) and can be hours. as short as 40 minutes, if 36 hours or more have been spent at the reduced 70.2-kPa (10.2-psia) cabin pressure. Shirtsleeve, Equilibration of tissue nitrogen tensions at a given ambient ground-based altitude chamber testing of these protocols 414,15 pressure is generally considered to require 36 hours; there- resulted in a DCS incidence rate of 23.7%, with most of fore, longer in-flight times spent at 70.2 kPa (10.2 psia) would the symptoms being minor joint pain. DCS has never been be expected to mitigate the decompression stress. Figure 5.4-2 reported during more than 140 space EVAs in which these illustrates predicted bubble growth for 12- to 36-hour exposures protocols were used, and the DCS rate has been less than at 70.2 kPa (10.2 psia);7 these theoretical calculations suggest 1.5% during more than 300 ground-based vacuum chamber that decompression stresses are very low after spending 24 tests, which were conducted while the subjects wore EMU hours at 70.2 kPa (10.2 psia).

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